The present invention is directed to the production of the anti-neoplastic compound paclitaxel. More particularly, the present invention is directed to the production of paclitaxel from a protected coupled ester intermediate, which may be formed by esterifying a protected baccatin III backbone with a suitably protected side chain acid. In particular, the present invention relates to the production of paclitaxel by esterifying 7-CBZ baccatin III with a 3-N-CBZ-2-O-protected-(2R,3S)-3-phenylisoserine to produce a protected coupled ester intermediate that may thereafter be deprotected and N-benzoylated to produce paclitaxel.
Various taxane compounds are known to exhibit anti-tumor activity. As a result of this activity, taxanes have received increasing attention in the scientific and medical community. Primary among these is a compound known as xe2x80x9cpaclitaxelxe2x80x9d which is also referred to in the literature as xe2x80x9ctaxolxe2x80x9d. Paclitaxel has been approved for the chemotherapeutic treatment of several different varieties of tumors, and the clinical trials indicate that paclitaxel promises a broad range of potent anti-leukemic and tumor-inhibiting activity. Paclitaxel has the formula: 
Paclitaxel is a naturally occurring taxane diterpenoid which is found in several species of the yew (genus Taxus, family Taxaceae). Unfortunately, the concentration of this compound in the yew is very low, and the species of evergreen are also slow growing. Even though the bark of the yew trees typically exhibit the highest concentration of paclitaxel, the production of one kilogram of paclitaxel requires approximately 16,000 pounds of bark. Thus, the long-term prospects for the availability of paclitaxel through isolation are discouraging.
While the presence of paclitaxel in the yew tree is in extremely low concentrations, there are a variety of other taxane compounds, such as baccatin III, cephalomanine, 10-deacetylbaccatin III, etc., which are also able to be extracted from the yew bark and leaves. Some of these other taxane compounds are more readily extracted in higher yields. Indeed, a relatively high concentration of 10-deacetylbaccatin III can be extracted from the leaves of the yew as a renewable resource.
Accordingly, attention has turned to the semi-synthesis of paclitaxel from precursor compounds. In order to successfully synthesize paclitaxel, convenient access to a chiral, non-racemic side chain acid and an abundant natural source of a usable baccatin III backbone as well as an effective means of joining the two are necessary. However, the esterification of the side chain acid to the protected baccatin III backbone is difficult because of the steric hindrance of the 13-hydroxyl which is located in the baccatin III backbone within the concave region of the hemispherically shaped baccatin III skeleton.
Some early synthetic routes in the semi-synthesis of paclitaxel are described, for example, in U.S. Pat. No. 5,770,745 to Swindell et al. The use of protecting groups to protect various positions of the taxane backbone and the side chain acid was investigated as a means of improving the chemical process to form paclitaxel, and of improving the esterification step in particular.
One technique for the semi-synthesis of paclitaxel is found in U.S. Pat. No. 5,750,737 to Sisti et al. As discussed therein, paclitaxel can be synthesized by joining 7-CBZ baccatin III of the formula: 
(where CBZ is the xe2x80x9cbenzyloxycarbonylxe2x80x9d group, xe2x80x94CO2CH2Ph), with 3-N-CBZ-2-O-protected (2R,3S)-3-phenylisoserine of the formula: 
where the 2-hydroxyl is protected by a hydrogenatable benzyl-type group P1 such as benzyloxymethyl (BOM) or benzyl. 7-CBZ baccatin III may be formed through the synthesis and use of 7-metal alkoxide intermediates and analogs of baccatin III, as described, for example, in U.S. Pat. Nos. 5,750,737 and 5,973,170 to Sisti et al. The production of the 3-N-CBZ-2-O-protected (2R,3S)-3-phenylisoserine is taught, for example, in U.S. Pat. No. 5,684,175 to Sisti et al.
Following the esterification of the protected baccatin III with the protected side chain to form a protected coupled ester of the formula: 
the compound may be suitably deprotected, acylated, and further deprotected to yield paclitaxel. Specifically, the CBZ protecting groups at the 7-O and 3xe2x80x2-N positions are removed, a benzoyl group is added at the 3xe2x80x2-N position and the 2xe2x80x2-O-protecting group is removed. U.S. Pat. No. 5,750,737 describes a deprotection and acylation sequence involving various steps to arrive at the final desired product. In particular, that patent teaches the use of work-ups involving recovery and purification steps (such as filtration, reduction to residue under vacuum, organic phase separation, and the like) in between the various steps. Furthermore, the hydrogenolysis of the coupled ester with Pearlman""s catalyst as described therein could take about one day to proceed to completion of the deprotection at the 7-O and the 3xe2x80x2-N positions by removal of the two CBZ groups. Additionally, after benzoylation of the 3xe2x80x2-amino group, the hydrogenolysis of the 2xe2x80x2-O-BOM paclitaxel took several days to complete, and included catalyst replacement as well as isolation and purification of the 2xe2x80x2-O-BOM paclitaxel intermediate. Additionally, factors such as preliminary purification of the 2xe2x80x2-O-BOM-paclitaxel intermediate as well as change of the catalyst and reaction medium contribute to high cost of the hydrogenation process.
While the existing techniques for synthesizing paclitaxel certainly have merit, there is still a need for improved chemical processes that can produce this anti-cancer compound and intermediates useful in the synthesis and semi-synthesis thereof. In particular, it is desirable to provide efficient processes requiring shorter times and fewer steps while still providing acceptable yields in the semi-synthesis of paclitaxel. Accordingly, the present invention is directed to an improved synthesis of paclitaxel or other taxanes from a protected coupled ester intermediate. The present invention teaches a new, useful and more efficient method for the conversion of the protected coupled ester to paclitaxel that may be performed in a single reaction vessel.
It is an object of the present invention to provide a new and useful method for synthesizing paclitaxel.
It is another object of the present invention to provide new intermediate compounds useful in the production of paclitaxel.
It is a further object of the present invention to produce paclitaxel from a protected coupled ester of the formula: 
which may be deprotected and N-acylated to yield paclitaxel.
It is yet another object of the present invention to provide methods for producing paclitaxel which are simplified and which may be suitable for large scale production of paclitaxel for anti-neoplastic applications.
It is yet another object of the present invention to improve the efficiency of the hydrogenolytic conversion of a protected coupled ester to paclitaxel.
It is yet another object of the present invention to convert a protected coupled ester to paclitaxel in a single vessel without isolation or purification of a 2xe2x80x2-O-protected paclitaxel intermediate.
According to the present invention, then, a method is provided of producing paclitaxel from a protected coupled ester compound having a formula: 
wherein P1 is a hydrogenatable protecting group, such as benzyl, substituted benzyl, benzyloxymethyl, or benzoyl. The method comprises the steps of deprotecting the 7-O-position, 3xe2x80x2-N-position, and 2xe2x80x2-O position of the protected coupled ester compound in the presence of an acid to form a first intermediate compound having a formula: 
wherein HA is the acid, and benzoylating the 3xe2x80x2-N-position of said first intermediate compound thereby to produce paclitaxel. The acid may be an inorganic or organic acid, and is preferably hydrochloric acid. The protected coupled ester compound is preferably dissolved in a solvent to form a first solution prior to the step of deprotecting the 7-O-position, 3xe2x80x2-N-position, and 2xe2x80x2-O-position of the protected coupled ester compound. The solvent may be one that includes an ether, ester, or alcohol functional group, such as THF, ethyl acetate, methanol or isopropanol. Water is preferably present in said first solution in from 10% to 25% (v/v) of said solvent, and 5 to 20 mol equivalents of the acid is preferably added to the first solution, along with a hydrogenation catalyst such as Pearlman""s catalyst or palladium on carbon catalyst, to form a first reaction mixture. The catalyst is preferably 10% Pd/C 50% wet, in an amount of 30% to 80% mass equivalent of said protected coupled ester.
The step of deprotecting the 7-O-position, 3xe2x80x2-N-position and 2xe2x80x2-O position of the protected coupled ester compound is accomplished by hydrogenolytic deprotection by stirring the first reaction mixture under a hydrogen atmosphere for 30 to 60 minutes.
The step of benzoylating the first intermediate compound may be accomplished by mixing benzoyl chloride and triethylamine with the first intermediate compound to form a second reaction mixture. Preferably, 1.20 mol equivalents of benzoyl chloride is mixed with the first intermediate compound, and, after addition of triethylamine, the second reaction mixture is stirred for 30 minutes under an inert atmosphere, such as a nitrogen atmosphere.
The present invention also relates to a method of producing paclitaxel, that comprises the steps of stirring a first reaction mixture including a solvent, an acid, a hydrogenation catalyst and a protected coupled ester compound having a formula: 
in a reaction vessel under a hydrogen atmosphere, adding a benzoylating agent, such as benzoyl chloride and triethylamine, to the reaction vessel to form a second reaction mixture and stirring the second reaction mixture, such as under an inert atmosphere, thereby to produce paclitaxel. A compound having the formula: 
and a compound having the formula: 
may be formed in the reaction vessel during the step of stirring the first reaction mixture under the hydrogen atmosphere.
Additionally, the present invention is directed to a chemical compound useful in the production of paclitaxel, having the formula: 
wherein P1 is a hydrogenatable protecting group and wherein HA is an inorganic or organic acid, such as hydrochloric acid, sulfuric acid, trifluoroacetic acid, ptoluenesulfonic acid, camphorsulfonic acid, and the like.
These and other objects of the present invention will become more readily appreciated and understood from consideration of the following detailed description of the exemplary embodiments and the accompanying figures, in which: